The present invention relates to the field of conditioning speech signals so that they may act as inputs to computerized systems and more particularly to the field of converting analog speech signals into digital data and vice versa, wherein clipped signal techniques are utilized.
When an analog signal is processed into a digital signal by a system which utilizes a clipping circuit, non-linearities are introduced because the clipping circuit is intrinsically non-linear. Non-linearities generally create harmonic contents in electronic signals which are difficult to filter out. In addition, non-linearities cause signal distortions which affect the fidelity of the information carried by the signal.
An article of interest for its general teachings in the field of present interest is: "Automatic Conditioning of Speech Signals" by G. A. Hellwarth et al., IEEE Transactions on Audio and Electroacoustics, Vol. AV-16, No. 2, pp. 169-179, June 1968;
A patent of interest is U.S. Pat. No. 3,278,685 entitled "Wave Analyzing System" by T. P. Harper, which illustrates the use of clipping circuits in conjunction with the digitizing of speech signals for purposes of voice recognition.
Another U.S. Pat. No. 3,071,652 entitled "Time Domain Vocoder" by M. R. Schroeder illustrates the further use of an infinite clipper in a system for band compressing a telephone message wave.
U.S. Pat. No. 4,015,088 entitled "Real Time Speech Analyzer" by J. J. Dubnowski et al. discloses another system wherein speech signals are conditioned utilizing clipping circuits.
In the article entitled: "Dither Signals and Their Effect on Quantization Noise" by L. Schuchman, IEEE Transaction Communication Technology, Vol. COM-12, pp. 162-165, December 1964. A general discussion of the effect of dither type signals on quantized signals is presented.
The present invention utilizes a dither signal to minimize the distortion in a digital signal representation of a speech signal which representation has been generated by passage through non-linear circuits.